1. Field of the Invention
The present invention relates, in general, to a drying apparatus and, more particularly, to a drying apparatus using a heat pump unit.
2. Description of the Prior Art
As well known to those skilled in the art, a drying apparatus has been used to easily store and transport agricultural products, marine products, livestock products or the like. As a heat source for the drying apparatus, fossil fuel, electricity, solar heat, ambient air or the like has been utilized.
A heat pump unit is a device that utilizes the ambient air as a heat source and operates a refrigeration cycle in a reverse manner. Since the heat pump unit employs the ambient air as a heat source, the evaporating efficiency of the coolant of its evaporator is in proportion to the temperature of the ambient air during the operation of the drying apparatus. Accordingly, the heat capacity of the heat pump unit is insufficient when the temperature of the ambient air is low. Many attempts have been made to overcome such a problem.
As an example, Japanese Utility Model Publication No. Sho 49-18927 discloses an air conditioning apparatus in which an outdoor heat exchanger and two indoor heat exchangers are disposed in a coolant cycle. This air conditioning apparatus is operated in such a way that during a room cooling operation, one of the indoor heat exchangers is utilized as an evaporator and the remaining one is shut to cool indoor air, while during a room heating operation, the outdoor heat exchanger is utilized as an evaporator and simultaneously the two indoor heat exchangers are all utilized as condensers to heat indoor air. Japanese Unexamined Patent Publication No. Sho 54-45949 discloses an air conditioning apparatus in which a coolant heater is disposed in a room heating cycle. The coolant heater is utilized as an evaporator only during a room heating operation, so that coolant liquid is forcibly evaporated in the coolant heater after high temperature and high pressure coolant gas compressed in a compressor is condensed and liquefied into the coolant liquid in an indoor heat exchanger to heat the room.
However, with regard to the former air conditioning apparatus, since the compressor must be utilized when indoor air is heated in the two indoor heat exchangers that function as condensers, the capacity of the compressor must be large, thereby increasing its manufacturing and maintenance costs. In addition, since two indoor heat exchangers must be mounted in the interior of a room, a large portion of the room is occupied by the air conditioning apparatus, thereby limiting the utilization of the interior of the room. Furthermore, since frost is deposited on the outdoor heat exchanger when the temperature of the ambient air is low during a room heating operation, the heat transmitting efficiency of the outdoor heat exchange is reduced, thereby reducing the evaporating efficiency of the coolant and, accordingly, causing the capacity of the air conditioning apparatus to be insufficient.
With regard to the latter air conditioning apparatus, it is described that the coolant is forcibly evaporated in the coolant heater, so its cooling capacity is not reduced and the defrosting of the outdoor heat exchanger is not necessary. However, since the construction of the coolant heater is not disclosed in the Publication, it is assumed that the coolant heater heats coolant using electric heat that allows the coolant heater to be small and to be easily mounted, thereby increasing the maintenance cost of the air conditioning apparatus.